1. Field of the Invention
The present invention relates to a fuel-cell system for purging residual water from a fuel cell and a method for scavenging a fuel cell.
2. Description of the Related Art
In general, a fuel cell includes a cathode zone and an anode zone, between which a proton-conducting solid polymer electrolyte membrane (PEM) is sandwiched. The cathode and the anode are supplied with air and hydrogen, respectively, and electricity is generated by electrochemical reaction between oxygen in air and hydrogen. In such a fuel cell, water is also generated as a result of electrochemical reaction between oxygen and hydrogen. During a cold season or in cold districts, if an operation of the fuel cell is terminated with the generated water remaining in the fuel cell and the fuel cell is not operated for a log period of time, the water may be frozen. In order to solve this problem, a method for scavenging a fuel cell has been known in which dry air is supplied to a fuel cell when an operation of the fuel cell is terminated, to thereby purge residual water from a fuel cell (see Japanese Patent Application Laid-Open Specification Kokai No. 2004-265684A, paragraphs 0030-0034 and FIG. 1).
According to this method, by scavenging an interior atmosphere of the fuel cell with dry air, residual water in the fuel cell can be suitably purged. In this method, an initial amount of the residual water is large, and the amount decreases as the purging proceeds. In response to the change in the amount of the residual water, it is considered to be desirable that a front-back differential pressure of the fuel cell (a difference in pressure between the inlet side and the outlet side, or a pressure drop) be controlled in such manner that the differential pressure initially becomes high and is gradually decreased, as shown in FIG. 4. Especially in this method, it is proposed that a flow rate of air supplied to the fuel cell be kept constant by controlling a compressor, in order to efficiently purge the residual water.
In the case of such a control in which an air flow rate is kept constant, a pressure drop becomes larger when an amount of residual water in the fuel cell becomes larger, leading to an increase in energy consumption in the compressor. There is a disadvantage in that the residual water is not efficiently purged, considering the increase in the energy consumption.
Therefore, it would be desirable to provide a fuel-cell system and a method for scavenging a fuel cell, in which residual water can be suitably purged from the fuel cell while energy consumption is suppressed.